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The Structure of the Atom

Chapter 4

+Important Dates for Chapter 4

Start: Tuesday October 8

End: Thursday October 24

Fall Break: October 14-15

Vocabulary Quiz: Thursday October 18

Test: Thursday October 24

Lab: Thursday October 18 and Tuesday October 22

+Essential Vocabulary Alpha particle

Alpha radiation

Atom

Atomic mass

Atomic mass unit

Atomic number

Beta particle

Beta radiation

Cathode ray

Dalton’s atomic theory

electron

Gamma ray

Isotope

Mass number

Neutron

Nuclear equation

Nuclear reaction

Nucleus

Proton

Radiation

radioactivity

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Early Theories of MatterSection 4.1

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The Philosophers

Democritus (460-370 B.C.) 1st person to propose the idea that

matter was not infinitely divisible. Believed atoms matter was made up

of tiny individual particles called atomos

Believed that atoms could not be created, destroyed, or further divided

Atoms are solid, homogeneous, indestructible, and invisible

Different kinds of atoms have different sizes and shapes

The differing of properties of matter are due to the size, shape, and movement of the atom

Apparent changes in matter result from changes in the groupings of atoms and not from changes in the atoms themselves

Most of his ideas do not agree with modern atomic theory, but his belief in the existence of atoms was amazingly ahead of his time

Many Greek philosophers thought matter was formed of air, earth, fire, and water. They also associated properties with each of the 4 basic components of matter. The pairings of opposite properties, such as hot and cold, and wet and dry, mirrored the symmetry and balance the philosophers observed in nature. These early nonscientific and incorrect beliefs were not completely dispelled until the 1800s.

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The Philosophers

Aristotle (384-322 B.C.) One of the most influential

philosophers Wrote extensively on many

subjects, including politics, ethics, nature, physics, and astronomy

Most of his writings have been lost through the ages

Criticized Democritus’s ideas of atomic theory

He rejected Democritus’s atomic “theory” entirely because it did not agree with his own ideas on nature

Did not believe that the “nothingness” of empty space could exist

Able to gain wide acceptance for his ideas on nature– ideas that denied the existence of atoms– for nearly 2000 years!

Concept of the atom was revived in 18th century, but it took another 100 years before significant progress was made

Many Greek philosophers thought matter was formed of air, earth, fire, and water. They also associated properties with each of the 4 basic components of matter. The pairings of opposite properties, such as hot and cold, and wet and dry, mirrored the symmetry and balance the philosophers observed in nature. These early nonscientific and incorrect beliefs were not completely dispelled until the 1800s.

+John Dalton (1766-1844) English schoolteacher

Marked the beginning of the development of modern atomic theory

Revived and revised Democritus’s ideas based upon the results of scientific research he conducted Advancements in science

allowed him to perform experiments that allowed him to refine and verify theories

Able to accurately determine mass ratios

Proposed his atomic theory in 1803

Dalton’s Atomic Theory All matter is composed of

extremely small particles called atoms

All atoms of a given element are identical, having the same size, mass, and chemical properties; atoms of a specific element are different from those of any other element

Atoms cannot be created, divided into smaller particles, or destroyed

Different atoms combine in simple whole number ratios to form compounds

In a chemical reaction, atoms are separated, combined, or rearranged

+Section 4.1 Assessment

Why were Democritus’s ideas rejected by other philosophers of his time?

Define an atom using your own words.

Which statements in Dalton’s original atomic theory are now considered to be incorrect? Describe how modern atomic theory differs from these statements.

Democritus and Dalton both proposed the concept of atoms. Describe the method each of them used to reach the conclusion that atoms existed. How did Democritus’s method hamper the acceptance of his ideas?

Compare and contrast the atomic theories proposed by Democritus and John Dalton.

+Subatomic Particles and the Nuclear AtomSection 4.2

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Discovering the Electron

Using a recently invented vacuum pump, electricity was passed through glass tubes from which most of the air and matter had been removed Electrode connected to negative

end is called anode; electrode connected to positive end is called cathode

Sir William Crookes English physicist noticed a flash of

light within one of the tubes while working in a darkened laboratory

Flash was produced by some form of radiation striking a light-producing coating that had been applied to the end of the tube

This led to the discovery of radiation traveling from the cathode to the anode within the tube; became known as a cathode ray

Now you have TVs and computer screens

Has your hair ever clung to your comb? Have you ever gotten shocked by a doorknob after walking across carpet? Observations such as these led scientists in the 1800s to look for some sort of relationship between matter and electric charge.

+Scientists continued their research using cathode ray tubes, and by the end of the 1800s, they were fairly convinced of the following: Cathode rays were actually a

stream of charged particles

Particles carried a negative charge (the exact value of the charge remained unknown)

Because changes in type of electrode or varying the gas did not affect the ray produced, it was concluded that negative particles were present in all forms of matter Called these negative

particles electrons

+ In spite of the progress

made from all of the cathode ray tube experiments, no one had succeeded in determining the mass of a single cathode ray particle

English physicist J.J. Thomson began a series of experiments in the late 1890s to determine the ratio of its charge to its mass

His conclusion was shocking because it meant that there were particles smaller than an atom; in other words, Dalton was wrong

The next significant discovery came in 1909 when American physicist Robert Millikan determined the charge of an electron to be 9.1x10-28g

Thomson then proposed a model called the plum pudding model which didn’t last long

+The Nuclear Atom Ernest Rutherford became

interested in studying how positively charged alpha particles interacted with solid matter

Aware of Thomson’s plum pudding model, Rutherford expected only minor deflections through the gold foil

After a few days of testing, Rutherford was amazed to discover that a few of the alpha particles were deflected at very large angles; some even deflected straight back

This meant the plum pudding model was incorrect; Rutherford concluded there must be a nucleus centrally located within the atom that contained the positive charge and most of its mass

Gold Foil Experiment

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Completing the Atom– The Discovery of Protons and Neutrons

In 1932, Rutherford’s coworker, English physicist James Chadwick showed that the nucleus also contained another subatomic particle, a neutral particle called the neutron. A neutron has a mass nearly

equal to that of a proton but carries no electrical charge

By 1920, eight years after his revolutionary gold foil experiment, Rutherford had refined the concept of the nucleus. He concluded that the nucleus contained positively charged particles called protons. A proton is a subatomic particle carrying a charge equal but opposite that of an electron.

+Section 4.2 Assessment Briefly evaluate the experiments that led to the

conclusion that electrons were negatively charged particles found in all matter.

Describe the structure of a typical atom. Be sure to identify where each subatomic particle is located.

Make a table comparing the relative charge and mass of each of the subatomic particles.

Compare and contrast Thomson’s plum pudding atomic model with Rutherford’s nuclear atomic model.

Make a timeline of the development of modern atomic theory. Be sure to include the discovery of each subatomic particle.

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How Atoms DifferSection 4.3

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Henry Moseley discovered atoms of each element contain a unique positive charge in their nuclei.

Thus, the number of protons in an atom identifies it as an atom of a particular atom and that number is referred to as its atomic number

Atomic Number Atomic number=

number of protons=Number of electrons

+Isotopes and Mass Number

Most elements are found as a mixture of isotopes found in a constant relative abundance

Those that contain more neutrons have a greater mass

Have essentially the same chemical behavior regardless of differing number of neutrons

To make it easy to identify each of the various isotopes of an element, chemists add a number after the element’s name This number is known as the mass

number and represents the number of protons and neutrons in the nucleus

# of neutrons = mass # - atomic #

Earlier you learned that Dalton’s atomic theory was wrong about atoms being indivisible. It was also incorrect in stating that all atoms of a particular element are identical. While it is true that all atoms of particular element have the same number of protons and electrons, the number of neutrons on their nuclei may differ. Atoms with the same number of protons but different numbers of neutrons are called isotopes.

+Mass of Individual Atoms

The masses of protons and neutrons are about 1.67x10-

24g each.

Electrons are very small– about 1/1840 that of one proton or neutron.

Because these extremely small masses expressed in scientific notation are difficult to work with, chemists developed a method of measuring based on a chosen atomic standard- a carbon-12 atom.

Thus, one atomic mass unit (amu) is defined as 1/12 the mass of a carbon-12 atom.

Because an atom’s mass depends on the number of protons and neutrons it contains, and because protons and neutrons have masses close to 1 amu, you might expect the atomic masses to be very near a whole number.

This does not hold true when the definition of atomic mass is taken into account. Atomic mass of an

element is the weighted average mass of the isotopes of that element.

+Section 4.3 Assessment Which subatomic particle identifies an atom as that of a

particular element? How is this particle related to the atom’s atomic number?

What is an isotope? Give an example of an element with isotopes.

Explain how the existence of isotopes is related to atomic masses not being whole numbers.

Nitrogen has two naturally occurring isotopes, N-14 and N-15. The atomic mass of nitrogen is 14.007 amu. Which isotope is more abundant in nature? Explain.

List the steps in the process of calculating average atomic mass given data about the isotopes of an element.

+Unstable Nuclei and Radioactive DecaySection 4.4

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Radioactivity

Nuclear reactions are reactions that do involve an atom of one element changing into an atom of another element.

In the late 1890s, scientists noticed that some substances spontaneously emitted radiation in a process they termed radioactivity.

The rays and particles emitted by the radioactive material were called radiation.

Unstable nuclei lose energy by emitting radiation in a spontaneous process called radioactive decay. Instability is caused by either

too many or too few neutrons.

Recall that chemical reactions involve the change of one more substances into new substances. Although atoms may be rearranged, their identities do not change during the reaction. You may be wondering why atoms of one element do not change into another element during a chemical reaction. The reason lies in the fact that the chemical reaction involves only an atom’s electrons- the nucleus remains unchanged.

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Types of Radiation

Alpha Radiation Deflected toward the

negatively charged plate Made up of alpha particles

2 protons, 2 neutrons, +2 charge

Beta Radiation Deflected toward the

positively charged plate Made of one fast moving

electrons called beta particles 1 electron, -1 charge

Gamma Radiation or Gamma Rays High-energy radiation that

possess no mass and no charge

Scientists began researching radioactivity in the late 1800s. By directing radiation from a radioactive source between two electrically charged plates, scientists were able to identify three different types of radiation. Some was deflected toward the negatively charged plate, some was deflected toward the positively charged plate, and some was not deflected at all.

+Section 4.4 Assessment Explain how unstable atoms gain stability. What

determines whether or not an atom is stable?

Create a table comparing the mass and charge of alpha, beta, and gamma radiation.

In writing a balanced nuclear equation, what must be conserved?

Explain how a nuclear reaction differs from a chemical reaction.

Classify each of the following as a chemical reaction, a nuclear reaction, or neither. Thorium emits a beta particle. Two atoms share electrons to form a bond. A sample of pure sulfur emits heat energy as it slowly cools. A piece of iron rusts.